Melting Behavior of Rimed and Unrimed Snowflakes Investigated With Statistics of Triple‐Frequency Doppler Radar Observations

Abstract

Comparing the reflectivity flux at the top and bottom of the melting layer (ML) reveals the overall effect of the microphysical processes occurring within the ML on the particle population. If melting is the only process taking place and all particles scatter in the Rayleigh regime, the reflectivity flux increases in the ML by a constant factor given by the ratio of the dielectric factors. Deviations from this constant factor can indicate that either growth or shrinking processes (breakup, sublimation, and evaporation) dominate. However, inference of growth or shrinking dominance from the increase in reflectivity flux is only possible if other influences (e.g., vertical wind speed) are negligible or corrected. By analyzing radar Doppler spectra and multi‐frequency observations, we correct the reflectivity fluxes for vertical wind and categorize the height profiles by the riming degree at the ML top. We apply this reflectivity flux ratio (ZFR) approach to a multi‐month mid‐latitude winter data set that contains mostly stratiform clouds. The profiles of radar variables in the ML are found to be surprisingly similar for both unrimed and rimed profiles with slight differences, for example, in the absolute values of the reflectivity flux. Statistical analysis of the ZFR suggests that either microphysical processes other than melting are not important or strongly compensate for each other. The results seem to confirm that at least for moderately precipitating stratiform clouds, the melting‐only assumption applied in several retrievals and microphysical schemes is reasonable.

Plain Language Summary: To better predict precipitation by numerical models and quantify precipitation by observations, it is important to improve the understanding of processes in the melting layer (ML). The ML is the part of clouds where ice particles melt and become rain. We use an approach that assesses whether a tendency toward either growth or shrinking processes is evident in the ML. We assess the uncertainty of the approach, correct for different factors, and apply it to a large data set to derive robust statistics separately for profiles with different characteristic ice particle shapes above the ML. These statistics are surprisingly similar for the different characteristic ice particle shapes and suggest that either growth and shrinking processes are not important in the ML or strongly compensate for each other.

Key Points: We investigated the growth or shrinking of snowflakes in the melting layer using statistics of multi‐frequency Doppler radar observations.

Reflectivity flux analysis indicates only slight differences for unrimed or rimed particles.

Growth or shrinking processes either compensate each other or have, on average, only a small impact on the reflectivity flux.